This invention pertains to closed loop microprocessor control of air-to-fuel mass ratio (A/F) in a fuel-injected automotive internal combustion engine using feedback signals from the vehicle""s exhaust oxygen (O2) sensor. More specifically, this invention pertains to a process for modifying the operation of the vehicle fuel control system to change the on-off, nonlinear response of the exhaust O2 sensor to a more useful proportional response around the stoichiometric A/F. The present invention is driven by the need to operate the engine slightly rich or slightly lean for their respective emissions advantages during hot or cold engine operation.
Most current production exhaust oxygen sensors (EOS) are zirconia-based, solid electrolyte, electrochemical devices that are used in conjunction with three-way catalytic converters to precisely control exhaust air-fuel ratio and to reduce vehicle tailpipe emissions. These robust sensors have been used for many years with powertrain control modules (PCM) for closed-loop computer control of fuel injector(s) in supplying gasoline to the cylinders of the engine in amounts near the stoichiometric A/F. The stoichiometric A/F is approximately 14.7 for most common gasoline fuels available in the market today. The PCM is programmed for engine operation near the stoichiometric A/F for best performance of the catalytic converter.
Such oxygen sensors are heated by exhaust gas or an additional power supply and produce a relatively low voltage (0.0 V to approximately 0.5 V) at A/F higher than stoichiometric value and a high voltage (0.5 V to 1.0 V) at lower A/F ratios. Around the stoichiometric A/F, the response of these sensors is unreliable and indeterminate in the manner in which they have been used. Therefore, the sensor steady state characteristic response to the exhaust mixture variations is unusable when the combustion mixture of air and fuel changes from slightly rich of stoichiometric A/F to slightly lean, or vice versa. In other words, in this region the magnitude of the voltage signal from the zirconia electrolyte oxygen sensor cannot presently be used by the PCM as indicative of the precise A/F. Instead, the O2 sensor is treated by the PCM as an on-off device, or simply as a fuel lean or fuel rich indicator (with respect to stoichiometric A/F). The PCM is programmed to respond, at a fixed rate, by changing the duration of the next fuel injection event to compensate for the present fuel lean or rich indicating signal.
Depending upon engine speed and the number of engine cylinders, the PCM must command a fuel injection amount into the intake port of a cylinder, or into a cylinder, many times per second. By considering the current and recent past O2 sensor voltage values to adjust the duration of the next fuel injection event, the PCM delivers an almost average stoichiometric A/F to the engine. However, because of the on/off nature of the sensor signal, the engine is essentially just cycling around stoichiometric A/F. Such an xe2x80x9caveragedxe2x80x9d A/F closed-loop control achieves significant reductions in emissions of unburned hydrocarbons, carbon monoxide and nitrogen oxides as compared to a pre-programmed PCM using the open-loop control and without actual feedback from downstream exhaust information. However, further reductions in tailpipe emissions are required in the recent government-mandated emission standards. Therefore, more precise A/F management and fuel control is now essential.
Accordingly, it is an object of this invention to provide a process for changing the switch-like output of the venerable zirconia-type oxygen sensor to a proportional response around the stoichiometric A/F. This newly created linear range in the output of the production O2 sensors would allow the current PCM, or fuel controller, to manage engine operation closer to the desired air-to-fuel ratio (slightly lean or rich of stoichiometric) and achieve further emission reductions with essentially the current production hardware.
The oxygen content in the combustion products of a hydrocarbon-fuel engine depends significantly on the proportions of air and fuel supplied to the cylinders. When the A/F is above the stoichiometric value, there is significant excess oxygen for combustion and, consequently, an appreciable oxygen content in the exhaust. The zirconia-based oxygen sensor operates with ambient air at one electrode and the oxygen containing exhaust at the counter electrode. Under such A/F conditions, the voltage response of a zirconia-based oxygen sensor is fairly constant in the range of about 0.0 V to about 0.5 V. When the engine is momentarily operated at A/F below the stoichiometric value, the excess oxygen is immediately utilized in combustion of hydrocarbon fuel and much less oxygen remains in the exhaust. The voltage output of the sensor is typically increased to the range of about 0.5 V to 1.0 V at all A/F above the stoichiometric value.
The voltage response of the sensor changes abruptly from one such range to the other when the engine A/F changes from slightly above the stoichiometric value to slightly below, or vice versa. This change in voltage is subject to process variability, and the sensor output in the range 0.3 V to 0.6 V cannot be reliably used by a PCM for fuel control. This range corresponds to the approximate critical A/F range of the stoichiometric value xc2x11.0 A/F (i.e., 13.7 to 15.7 for gasoline fuel).
In accordance with this invention, it has been found that by continually introducing a suitable pattern of individual fuel injector biases of known size, at any given engine speed, high frequency A/F oscillations of desired amplitude are produced at the oxygen sensor location. The PCM controls the on time of some or all of the injectors to deliver amounts of fuel that deviate from the average amount prescribed by the current PCM determined fueling strategy. Such deviations are imposed in each engine fueling cycle in which it is desired to operate the engine in accordance with this invention. The imposition of these individual cylinder A/F imbalances, through fuel injector biasing, changes the on-off nonlinear characteristic of the O2 sensor in the affected A/F range. The result is a modification of the steady state characteristic of the sensor so that a dependable proportional response of the O2 sensor over an A/F range of, e.g., 14.7xc2x10.5 is created.
For example, in a 4-cylinder engine, with the cylinder firing sequence 1-3-4-2, running at a steady 1500 rpm, fuel injection events occur at 20 millisecond (ms) intervals or with a frequency of 50 Hz. On selected fuel injectors (2 or 4 injectors), the fuel pulse widths are altered to cause a slight perturbation of A/F. During such steady engine operation (e.g., at 60 kPa), a normal fuel injection period per cylinder of about 6.0 ms may provide near stoichiometric A/F. Instead, injection duty cycles of 6.6 ms, 6.0 ms, 5.4 ms and 6.0 ms in cylinders 1, 3, 4, and 2, respectively, are repeatedly introduced in the cylinder intake ports or directly into the cylinders.
These and like fuel imbalances, preferably introduced during each fueling cycle of engine operation, produce a fluctuation in exhaust oxygen content and, thus, in voltage output at each event. The imbalances, when introduced, will typically amount to about one to fifteen percent of the amount of fuel that the PCM determines to be injected in the next cylinder fueling event to maintain the desired A/F. Averaged values of these outputs provide a usable linear voltage response to changes in A/F. The magnitude of the imbalance in injection time, here, e.g., xc2x10.6 ms, produces a proportional range in the downstream oxygen sensor response characteristic around the stoichiometric A/F. Such alteration of the nature of the oxygen sensor characteristics over the selected A/F range is suitably utilized to achieve the desired A/F as follows.
In a family of substantially identical production engines, such as four-cylinder intake port fuel-injected engines, the PCM is programmed to time the fuel injector duty cycles over a full range of operating conditions of the engine. In accordance with this invention, a calibration curve is obtained for correlation between the average sensor voltage and the average exhaust pipe A/F measured in a test environment on a representative engine. This calibration curve characteristic depends on the magnitude of fuel injector biasing and the pattern imposed as well as other operating conditions such as airflow rate and engine speed. Suitable nominal values for fuel injection duration are selected for test operation of a representative engine for adaptation of the subject process. A pattern and level of individual fuel injector biasing is initially selected. An oxygen sensor reference voltage, for example, Vs =0.5 V, is selected and fuel injection is controlled, with the selected cylinder fuel flow and biasing pattern, to operate the engine at pre-converter sensor location Vs values around the chosen level. One can suitably start at 0.5 V and proceed higher or lower in small increments and slowly enough to allow steady state operation at each Vs level established. While the test engine is being operated at selected average Vs levels, air to fuel ratios based on tailpipe exhaust analyses, downstream of the catalytic converter, are being carefully measured.
The tailpipe analysis may be performed with a suitable wide-range A/F exhaust sensor or, preferably, with an emission gas analyzer for more precision. This exhaust data is used to correlate the average of measured A/F with the corresponding average voltage responses of the pre-converter production O2 sensor exposed to the exhaust stream from the perturbed fuel injector operation. This practice is suitably repeated over a range of O2 sensor set points, for example, from 0.1 to 0.9 volt.
Such responses of the O2 sensor are essentially proportional to the average exhaust A/F over a small region around the stoichiometric A/F. The sensor proportional range created in this way is equal to the magnitude of A/F biasing imposed in the cylinders. For example, an injector biasing of xc2x10.5 A/F between cylinders would lead to an extended proportional range of xc2x10.5 A/F in the sensor characteristic around the stoichiometric A/F value. This proportional range may, however, be affected by significant mixing in the exhaust pipe before reaching the O2 sensor location. For this reason, the sensor is installed at a location very close to the exhaust port where cylinders merge.
The data produced is then stored in the PCM memory in the form of an O2 sensor characteristic lookup table for future engine control. The PCM can now command fuel injections to the respective cylinders using a suitable pattern and level of injector biasing to produce a limited linear exhaust oxygen sensor (LEOS) response from an otherwise highly nonlinear device. In the case of each cylinder fueling event, the bias is from the amount of fuel, or injector on-time, determined for that cylinder in view of current vehicle speed and power requirements. And the PCM can refer to this lookup table to control the overall injection cycles so that the appropriate O2 sensor response is maintained and, as a result, the desired A/F is achieved.
This process can be adapted for use during cold starts to reduce emissions of unburned hydrocarbons and carbon monoxide, or it can be used for slightly fuel rich for warmed-up or hot engine operation to markedly reduce NOx emissions.
Other objects and advantages of the invention will become more apparent from a description of a preferred embodiment which follows. Reference will be made to the drawing figures which are described in the following section.